This invention relates to a supercharging device for supercharging an internal combustion engine to increase an output level and, in particular, to a supercharging device comprising a mechanical supercharger operated at a rotation speed controlled by a continuously variable transmission of a centrifugal weight type.
A conventional mechanical supercharger operated at a rotation speed controlled by a continuously variable transmission (often abbreviated to CVT) of a centrifugal weight type is disclosed in Japanese Patent Prepublication No. 500564/1991. In this supercharger, the continuously variable transmission is driven by the internal combustion engine. A runner shaft with a runner is rotatably supported on a housing and driven by a secondary pulley (driven pulley) of the continuously variable transmission through an electromagnetic clutch and a speed increasing gear train. Thus, the internal combustion engine is supercharged. The above-mentioned Japanese Patent Prepublication contains no disclosure about a primary pulley (drive pulley) of the continuously variable transmission. An applicable arrangement of the primary pulley and the secondary pulley is disclosed, for example, in Japanese Utility Model Prepublication No. 69400/1989. In this arrangement, each of the drive pulley and the driven pulley comprises a fixed sheave (disk) and a movable sheave faced to each other. The fixed sheaves of the drive pulley and the driven pulley are arranged on the opposite sides. The movable sheaves serve to vary the widths of V-shaped grooves of the drive pulley and the driven pulley. Either one of the pulleys has a weight for generating a centrifugal force and a spring for generating an urging force against the centrifugal force. Within a predetermined range of load, the conventional supercharger is controlled by the continuously variable transmission to be kept at a substantially constant rotation speed irrespective of variation of an engine rotation speed. Only in a high load condition which requires a supercharging operation, the supercharger is operated by power transmission through the electromagnetic clutch to improve performance of the internal combustion engine.
Once a certain engine rotation speed is reached, the conventional mechanical supercharger with the CVT of a centrifugal weight type is controlled to be kept at a predetermined constant rotation speed irrespective of variation of the load imposed on the internal combustion engine. In this connection, the supercharger reaches such a rotation speed (high-speed rotation) even under a partial load condition which requires no supercharging operation. To drive such a high-speed rotation, fuel consumption is increased. This results in decrease of a fuel efficiency. In view of the above, the electromagnetic clutch is used to cut off power transmission to the supercharger under the partial load condition. However, drivability is unfavorably affected by on/off operation responsive to variation of the load. Taking the above into consideration, a cutoff range inevitably becomes small. After all, the fuel efficiency is decreased.